Parkinson's disease is a progressive neurodegeneration of the melanized dopaminergic neurons in the substantia nigra. It is clinically characterized mainly by akinesia, bradykinesia and tremor at rest. Postmortem studies on brains from parkinsonian patients suggest the involvement of oxygen free radical-induced oxidative stress which results in lipid peroxidation of cell membranes, followed by increased membrane fluidity and finally cell death.
Normally dopamine (DA) is metabolized by either monoamine oxidase or by autooxidation. Both ways lead to an excess of toxic oxygen species, such as H2O2, which in the presence of a transient metal, such as iron, will produce cytotoxic oxygen free radicals, e.g. superoxide and hydroxyl free radicals. The brain, like all other tissues, protects itself against the deleterious effects of oxygen free radicals by specific protective enzymes such as glutathione peroxidase, catalase and superoxide dismutase, and by relatively high amounts of glutathione and ascorbate. In addition, iron is bound to high molecular weight proteins such as ferritin, hemosiderin and transferrin, or to low molecular weight molecules such as ADP, ATP, catechol and probably also melanin, and its amount in the brain is strictly conserved by the blood brain barrier (BBB).
In Parkinson's disease, the brain defensive mechanisms against the formation of cytotoxic oxygen free radicals are defective. In the substantia nigra of parkinsonian brains there are reductions in activities of superoxide dismutase and glutathione peroxidase and reduced tissue contents of glutathione and ascorbate. Moreover, iron concentrations are significantly elevated in parkinsonian substantia nigra pars compacta within the melanized dopamine neurons. These conditions favor liberation of free cytotoxic radicals, which can cause among other things release of intracellular calcium and lipid peroxidation resulting in neuronal death. Indeed an increase in basal lipid peroxidation in the substantia nigra of parkinsonian patients has been detected.
Iron alone or iron decompartmentalized from its binding site by a neurotoxin, e.g. the dopaminergic neurotoxin 6-hydroxydopamine (6-OHDA), may induce oxidative stress and neurodegeneration, as evidenced in previous studies of the inventors in which intranigral administration of iron induced “Parkinsonism” in rats and the iron chelator desferrioxamine protected the rats against 6-OHDA-induced lesions of nigrostrial dopamine neurons (D. Ben-Shachar and M. B. H. Youdim, 1991, J. Neurochem. 56: 1441-4). It has thus been suggested that treatment or retardation of the process of dopaminergic neurodegeneration in the substantia nigra may be affected by iron chelators capable of crossing the blood brain barrier in a fashion similar to chelators used in the treatment of Wilson's disease and iron overload in systemic organs.
This may be a new therapeutic approach for the treatment of Parkinson's disease that can be applied to other metal-associated neurological disorders such as tardive dyskinesia, Alzheimer's and Hallervorden-Spatz diseases.
Stroke is the third leading cause of death in the western world today, exceeded only by heart diseases and cancer. The overall prevalence of the disease is 0.5-0.8% of the population. Stroke is characterized by a sudden appearance of neurological disorders such as paralysis of limbs, speech and memory disorders, sight and hearing defects, etc., which result from a cerebrovascular damage.
Haemorrhage and ischemia are the two major causes of stroke. The impairment of normal blood supply to the brain is associated with a rapid damage to normal cell metabolism including impaired respiration and energy metabolism lactacidosis, impaired cellular calcium homeostasis release of excitatory neurotransmitters, elevated oxidative stress, formation of free radicals, etc. Ultimately these events lead to cerebral cell death and neurological disfunction.
Treatment of stroke is primarily surgical. Much effort is being invested in less aggressive therapeutical intervention in the search for drugs which are capable of restoring normal blood perfusion in the damaged area as well as drugs which are designed to overcome the above listed damaging events associated with cellular damage.
Oxidative stress and free radical formation play a major role in tissue injury and cell death. These processes are catalyzed by transient metal ions, mainly iron and copper. In the case of stroke, since vascular damage is involved, iron is available for the free radical formation, a process that could be prevented by iron chelators. Indeed, with lazaroides (21-amino steroids), known free radical scavengers, a significant improvement of local and global ischemia damages induced in animals has been achieved.
For the treatment of Parkinson's disease and probably other metal-associated neurological disorders and for the treatment of trauma and stroke and the secondary injuries which follow them, it would be highly desirable to find neuroselective iron chelators that cross the blood brain barrier.